This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations employing a mixture of a biopolymer and a multicomponent surfactant system for mobility control.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
It has long been recognized that factors such as the interfacial tension between the injected water and the reservoir oil, the relative mobilities of the reservoir oil and injected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the reservoir rock. Also, it has been proposed to add viscosifiers such as polymeric thickening agents to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and improving the sweep efficiency of the waterflood. For example, a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Technology, Vol. 25, Feb. 1973, pp. 205-210, describes a promising technique involving the injection of an aqueous solution of petroleum sulfonates within designated equivalent weight ranges and under controlled conditions of salinity. The petroleum sulfonate slug is followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide a maximum viscosity greater than the viscosity of the reservoir oil and a terminal viscosity near that of water. This thickened water slug is then followed by a driving fluid such as a field brine which is injected as necessary to carry the process to conclusion.
One limitation encountered in waterflooding with anionic surfactants such as petroleum sulfonates is the tendency of the surfactants to precipitate from solution in the presence of even moderate concentrations of divalent metal ions such as calcium and magnesium ions. Thus, as taught for example in the Foster paper, the surfactant slug may be preceded by a protective slug which functions to displace the reservoir waters ahead of the subsequently injected surfactant slug. Another limitation imposed upon the use of anionic surface-active agents resides in the fact that desired low interfacial tensions can seldom be achieved, even in the absence of divalent metal ions, at salinities significantly in excess of 2 or 3 weight percent. Thus, the protective slug and the surfactant slug normally exhibit a relatively low salinity.
In view of the limitations imposed by the salinity characteristics of many reservoir waters and field waters available for injection purposes, various surfactant systems have been proposed for use in moderately to highly saline systems which may or may not contain significant quantities of divalent ions. For example, U.S. Patent Application Serial No. 560,289, filed Mar. 20, 1975 by Silvia C. Birk, discloses a low tension waterflooding process employing an anionic-nonionic surfactant system capable of achieving low oil-water interfacial tensions in systems exhibiting salinities within the range of 3 to 25 weight percent and in the presence of divalent ions ranging as high as 3 weight percent. The multicomponent surfactant system employed in the Birk process comprises an amide-linked sulfonate such as a disubstituted taurate containing a C.sub.12 to C.sub.20 aliphatic group and a nonionic surfactant component such as a polyethoxylated alkyl phenol or a polyethoxylated aliphatic alcohol.
As noted previously, it is a common expedient to employ polymeric thickening agents in a waterflood process in order to provide for mobility control. Many polymeric thickening agents have been proposed for use in this regard. One well known group of polymers which may be employed is the polysaccharide produced by the action of the genus Xanthomonas on carbohydrates. For example, U.S. Pat. No. 3,908,760 to Clampitt et al. discloses the use of such polysaccharides in the presence of polyvalent metal ions which act as complexing agents. Another thickened waterflooding process employing Xanthomonas polysaccharides is disclosed in U.S. Pat. No. 3,801,502 to Hitzman. In this process the viscosity yield of the polysaccharide thickening agents is increased by employing an additive selected from the class consisting of certain alcohols, ketones, phenols, and nonionic surfactants such as ethoxylated alcohols and ethoxylated alkyl phenols.